In a thermal printer with a thermal head having a plurality of heating elements and a transport mechanism that causes a relative movement of a thermally sensitive material with respect to the thermal head, the printing process is typically divided into print cycles whereby in each print cycle the heating elements receive different amounts of energy appropriate to cause the wanted densities on the part of the media that is in contact with the thermal head during that cycle.
The thermally sensitive material maybe composed of a donor sheet and an acceptor sheet as in the diffusion transfer process or maybe film or sheet of paper that is thermally sensitive by itself.
The present invention relates in particular to a printing method to be used in a thermal printer intended for applications that require high image quality such as printers for medical diagnosis.
In such a printer, images are supplied by a host computer, hereafter called “host-images” that have digital values directly or indirectly corresponding to the wanted density of a certain area on the printed media. This area is called a “host-pixel” further on. In case of a colour printer, each value corresponds directly or indirectly to the density of one specific colorant of the colour image. For each “host-pixel” and for each colour printed, one value is required.
The host-image is typically organised in rows and columns, whereby in one specific case each column corresponds to one heating element of the head and each row corresponds to one print cycle of the printing process. In all other cases, the source image has to be transformed into a new image with a different number of values so that each value corresponds to a heater element in a specific print cycle. This transformation is well known as “interpolation”.
If the resulting pixels of the printer-image are not directly corresponding to the densities wanted on the media, there is usually a translation mechanism that translates the “user” meaning of the values into a value that corresponds to the wanted density effect on the media (see FIG. 1).
Because the media can only be influenced by temperature of the heating elements, the density values must be translated into a temperature value for the heating elements. This is done using the sensitometry information of the media. This sensitometry information is obtained usually during a media calibration before using such a printer. Most printers have automatic means to perform a media calibration when entering new media into the printer.
At this point, the image is available as “wanted temperature” for each heating element during each heating cycle.
The only driving parameter the printer has available is the power it injects into each heating element during each heating cycle.
The amount of power that a printer needs to reach the wanted temperature for a certain density is highly dependent on different temperatures in the system. Therefore, in state of the art printers each printing cycle a line of the wanted temperature image is fed into a ‘thermal model’ along with measured temperatures and a state variable to calculate a line of power values which then are used to drive the heating elements. The State variable is a set of values that assist the thermal model to compensate for the temperature rise in the thermal head and the lateral heat distribution (see FIG. 2). An example of such a thermal model can be found in European patent application 671 276.
This thermal model has to run real-time and should therefore not be complicated, especially if it runs on a computer that has several other tasks to perform.
Without this thermal model, thermal printers would render very poor image quality and be unstable in density reproduction. Using a thermal model, state of the art printers are capable of producing images that are acceptable for medical application.
Most state of the art printers have an additional correction of the power-values to correct for the different behaviour of the individual heating elements (FIG. 3). This additional correction prevents uneveness along the thermal head due to variation in element properties such as resistance values of the heating resistors.
Although the quality of thermal printers is accepted, images of wet laser printers compared to state of the art thermal printers are still easier to read.
The cause of this effect is explained by means of an experiment illustrated in FIG. 4.
Suppose a density profile corresponding with a step function is printed, both on a state of the art thermal printer and on a laser printer and then the printed result is measured using a scanning micro densito meter.
In FIG. 4 curve A represents the wanted density for a transition from low to high density in transport direction, curve B is the density curve obtained by means of a typical wet laser printer, curve C is the resulting density curve of a typical state of the art thermal printer. Curve C is the result of the printer behaviour combined with the sensitometric transformation characteristic of the recording material.
Curve B of the laser printer has a typical shape due to the Gaussian beam distribution of the laser spot used to write the image. Already at a distance equal to a few times the spotsize of the laser from the edge, the density profile has reached its final value.
Curve C pertaining to the thermal printer clearly shows that at the first 2 mm after increasing the wanted density, the thermal model is not capable to predict the needed amount of power correctly.
FIG. 5 pertains to the direction parallel with the thermal head. Curve A represents the wanted density for a transition from low to high density in a direction parallel with the thermal head. Curve B is the resulting density curve of a typical wet laser printer. It is basically the same as the laser curve in the transport direction. Curve C is the resulting density curve of a typical state of the art thermal printer.
Curve C pertaining to the thermal printer clearly shows that in the first 1 mm distance from the edge of the low to high density transition, the thermal model is not capable to predict the needed amount of power correctly.
From these measurements can be concluded that some sharpness is lost due to the fact that the dynamics of heat storage in the head and the lateral heat spread along the head are not accurately modelled in the thermal model.
A way of solving this problem could be to simulate the heat distribution in the head using a Finite Element Model that is capable of predicting the temperatures in the neighbourhood of the heating elements very accurately and therefore also enables a computer system to calculate the required power to heat the elements to a desired value to any degree of accuracy.
Although a FEM model is capable of performing the task, a very expensive computer would be needed to run it at sufficient speed to serve as part of the thermal model in the printer.